The term “conveyor chain” in the present context refers, for example, to an endless-loop conveyor device analogous to a conveyor belt, but with the difference that a conveyor chain includes a multitude of rigid segments or links which are connected to each other in a closed loop wherein each link is articulately hinged to a following link and a preceding link. The segments can either be all identical to each other, or a group of dissimilar segments can identically repeat itself around the conveyor chain. The individual segment or group of segments that identically repeats itself is referred to herein as a module or as modular segment and, consequently, the conveyor chain is referred to as a modular conveyor chain.
The radiation transmittance of the endless-loop conveyor comes into play in inspection systems whose geometric arrangement is such that at least part of the scanner radiation passes not only through the products under inspection and the air space surrounding them, but also traverses the endless-loop conveyor. This kind of inspection system is used for example for the detection of foreign bodies in bottled or canned food and beverage products. Of particular concern are metal and glass fragments in liquid products. Due to their higher density relative to the liquid, such foreign bodies will collect at the bottom of the container. Furthermore, if the container has a domed bottom, the foreign bodies will tend to settle at the perimeter where the bottom meets the sidewall of the container. It is therefore very important in such systems for the radiographic scanner system to be configured and arranged in relation to the endless-loop conveyor in such a way that an entire inside bottom surface of each container is covered by the scan. Consequently, a scanner arrangement is used where at least part of the radiation passes through the bottom of the container and therefore also through the area of the endless-loop conveyor on which the container or any other object to be inspected is positioned.
In known arrangements, the radiation used for the inspection may for example originate from a radiation source located above the conveyor path, pass at an oblique angle through the sidewall into the container, exit through the container bottom and pass through the conveyor, to be received by a detection system which is connected to an image-processing system. Alternatively, for example when objects are inspected, that are neither bottled nor canned, the radiation source can be arranged vertically above the conveyor and the radiation detector vertically below said conveyor.
If the radiographic inspection system is an X-ray system, the rays can be received for example by an X-ray image intensifier and camera, or by an X-ray line array sensor, or by an X-ray area array sensor, both of which then pass a signal to the image processing system. For example, the imaging radiation originates as a fan-shaped planar bundle of rays from a localized radiation source (e.g., a spot-sized radiation source) and is received by a linear array of photodiodes that are collectively referred to as a radiation detector, wherein the fan-shaped radiation bundle and the linear array of photodiodes lie in a common plane, also referred to as the scanning plane, which runs substantially orthogonal to the travel direction of the conveyor carrying the articles to be inspected. While the articles under inspection move through the scanning plane, the linear array of photodiodes is triggered by a continuous sequence of discrete pulses, and the pulse frequency is coordinated with the speed of the conveyor so that the sequence of signals received by the radiation detector array can be translated into a pattern of raster dots with different brightness values expressed for example in terms of a brightness scale from zero to 255, representing a transparent shadow image of the material bodies between the radiation source and the radiation detector. If a scanned article contains foreign objects such as metal fragments, which have a lower transmittance to the scanning rays than the scanned article, the radiographic image will show such foreign objects as darker areas within the transparent shadow image of the scanned article.
At the present state of the art, endless-loop conveyors that are used as transport devices in radiographic inspection systems are for example, polymer fabric belts. This type of conveyor has an advantage that the quality of the X-ray image is least affected by it, due to the constant thickness and the uniformity of the belt. However, there are a number of strong arguments against polymer fabric belts and in favor of modular conveyor chains, specifically:
There is strong resistance to the use of fabric belts particularly in the bottling and canning industry, because they are easily damaged and wear out rapidly. In comparison, conveyor chains consisting of rigid plastic elements (such as of acetal resin or polypropylene) that are linked together in an endless loop are much stronger and less easily damaged by hard metal or glass containers.
Conveyor chains are better suited for heavy-weight articles such as blocks of cheese, as it is possible to drive the conveyor chain with sprockets that directly engage the chain profile.
The segments of a conveyor chain can be hinged together in such a way that the chain has a unilateral flexibility to loop around the drive sprockets while being rigid against bending in the opposite direction. This latter property eliminates the need for guiding mechanisms which can be unreliable in continuous-duty applications.
Conveyor chains are easier to replace or repair than belts, because the chain can be opened by removing one of the hinge pins by which the modular elements of the chain are linked together.
Conveyor chains can be designed to be self-tracking and to run flush with the sides of the conveyor support structure. This last characteristic can be important, because it can allow products to be easily transferred sideways between laterally adjacent conveyors.
Nevertheless, the use of customary chain conveyors with plastic chain links can be an issue in radiographic inspection systems, because the chain links can interfere with the X-ray image. Until now, if one wished to X-ray a product moving on a conveyor chain, the resultant image was degraded by the variations in the transmittance of the conveyor chain superimposed on the product, for example due to hinges or other connections between the chain segments or by profile features designed to stiffen the chain segments. If this issue of image interference can be solved, the benefits of modular conveyor chains as listed above can be applied to radiographic inspection systems.
In US 2012/0128133 A1, which is owned by the same assignee as the present disclosure, the issue of transmittance variations is solved through a conveyor chain in which the chain segments are configured in essence as rigid plates of uniform thickness and density extending over the width of the conveyor chain, wherein the segments overlap each other to present themselves to the scanner radiation as a substantially gapless band of uniform transmittance and wherein the connectors or hinges which link the segments together (and which have a lower transmittance than the flat areas of the segments) are located outside the band that is traversed by the scanner radiation. Thus, the connections between the segments are for example located in the two lateral border areas of the conveyor chain.
In a conveyor chain according to the foregoing concept, the absence of hinges or any other stiffening features in the central homogeneous band area reduces the rigidity of the chain segments in regard to transverse bending and therefore limits the conveyor width that can be realized in a practical design. A treatment for background effects in the radiographic image in state-of-the-art inspection systems is already known to the extent that such background effects are caused by the variation in the dark signal and gain between the individual photodiodes that make up a linear-array radiation detector. Part of this variation is due to random differences in the properties of the diodes themselves, while another part is due to the different lengths of the ray paths which radiate fanlike from a localized radiation source to the individual photodiodes. According to the inverse-square law, the shorter the ray path from the radiation source to a given sensor diode, the stronger the radiation received by that diode. In order to create a uniform image across the scanned area, the effect of these variations is cancelled through a so-called radiation detector calibration or radiation detector normalization.
In the first of two steps of the radiation detector calibration, the dark signals of the individual photodiodes of the radiation detector array are determined by measuring their respective diode currents while the radiation source is turned off. The respective brightness values for the dark signal of each diode are stored in memory. Subsequently, in the operating mode of the inspection system, the stored dark-level value is subtracted from the signal of each diode, so that the dark level of each diode corresponds to a net signal of zero.
In the second step of the radiation detector calibration, the signals of the individual diodes of the radiation detector array are determined with the radiation source turned on and with nothing but the empty conveyor belt located in the raypath between the radiation source and the radiation detector. These signals represent the respective maximum brightness level for each individual diode. Each signal is digitized, the respective dark level value is subtracted, and the resultant net signal values are used to calculate and store normalizing factors for the individual diodes. As a result of the calibration, the normalized dark level signal values for all diodes will all be zero, and the maximum brightness levels of all photodiodes will all correspond to an identical normalized value, for example 255 if expressed in terms of an 8-bit binary number.
In the operating mode of the radiographic inspection system, the raw measurement value from each diode is first converted into a net value by subtracting the stored dark signal value, and then into a normalized value by multiplying the net value with the stored normalizing factor for the respective diode.
As the calibration is performed with the conveyor in place, the reduction in brightness due to the conveyor is automatically included in the normalized results, so that in the case of a conveyor belt, a constant gray-level background in the radiographic image which is caused by the radiographic absorption of the belt is already cancelled out in the normalized brightness values.